The advent of heat-sealable plastic coatings, such as polyethylene, has given rise to the formation of various articles composed of superimposed panels that are heat-sealed together simultaneously with a cutting process. For example, U.S. Pat. No. 4,077,588 discloses toy balloons that are generally formed in the above manner. However, conventional apparatus and methods for forming articles of this type exhibit various problems, such as the inability to maintain the superimposed panels in precise alignment and to continuously ensure efficient sealing of the panels together to prevent subsequent leakage.